As investigators sought to decipher the motives of the gunman who targeted U.S. troops in Chattanooga, Tennessee, they also began to confront the uncomfortable question of whether counterterrorism agencies are reaching the practical limits of what they can do to detect homegrown plots.

On Friday, federal officials said they were investigating the Chattanooga shootings as a possible terrorist attack but were a long way from drawing conclusions. They said the gunman, 24-year-old Mohammad Youssef Abdulazeez, had not previously drawn the attention of authorities, save for a drunken-driving charge a few months ago.

But Abdulazeez's travels to the Middle East, his acquisition of multiple firearms and his recent online musings about the meaning of Islam were coming under fresh examination as hundreds of federal agents sought to reconstruct his movements and mind-set.

"At this time, we have no indication he was inspired by or directed by anyone other than himself," Edward Reinhold, the special agent in charge of the FBI's office in Knoxville, Tennessee, told reporters Friday.

U.S. officials said that devices including a computer and cellphone believed to have belonged to Abdulazeez were being examined by FBI technicians in a laboratory at Quantico, Virginia.

The FBI said that Abdulazeez was armed with at least two rifles or shotguns, as well as a handgun, when he opened fire on a military recruiting center and a Navy Reserve facility in Chattanooga. Authorities did not give a more detailed description of the firearms or say how he obtained them.

"Some of the weapons were purchased legally and some of them may not have been," Reinhold said.

U.S. counterterrorism officials have become increasingly worried about the ability of the Islamic State and al-Qaida offshoots to attract and radicalize followers in the United States. At the same time, authorities have expressed concern that their ability to detect such contact has been eroded by the spread of encrypted communication.

Federal authorities have arrested more than 10 people over the past six weeks who are suspected of having ties to the Islamic State. U.S. officials said the crackdown was part of an effort to suppress a surge in suspected plots aimed at unleashing violence on U.S. targets during the Muslim holy month of Ramadan, which ended on the day of the attacks in Chattanooga.

But officials have also said that homegrown radicals have gotten better at hiding their intentions and cloaking their contacts with overseas groups, despite a massive expansion in U.S. surveillance capabilities since the Sept. 11, 2001, attacks.

Abdulazeez traveled to Jordan last year and remained abroad for several months, according to U.S. officials, who said his travels at the time did not raise any red flags. Jordan has been a way station for foreign fighters seeking to enter Syria, including a 22-year-old U.S. citizen who similarly went undetected during trips to Jordan before carrying out a suicide attack in Syria last year.

But Jordan is also a popular tourist destination, one of several nations bordering Syria that account for more than 2 million travelers who arrive in the United States each year.

Moreover, Abdulazeez had a grandmother and other relatives in the country, according to neighbors and court papers.

And while his father, Youssuf Abdulazeez, was investigated by the FBI in 1994 and again in 2002 for donating to Palestinian groups suspected of ties to terrorism, U.S. officials said the father was removed from a terrorism watch list a decade ago.

Based on the limited information available so far, the younger Abdulazeez appears to have repeatedly brushed up against U.S. screening systems without ever triggering an alert, said Rep. Devin Nunes, R-Calif., chairman of the House Intelligence Committee.

"You have to do something that would set off some type of alarm," Nunes said in an interview. Because of the mounting odds against disrupting plots, as well as the countermeasures being taken by terrorist groups, Nunes said that stopping them is "becoming tougher and tougher."

Nunes said the FBI has warned lawmakers repeatedly in recent months that the bureau was facing a surge in the number of threats it is tracking — many based on intelligence gleaned overseas — but has been unable to connect those tips to individuals or specific targets in the United States.

"The FBI has warned us that there are a bunch of threats that they know about but can't find," Nunes said. "They have enough specifics to say something is being planned. We know [the Islamic State] is talking to someone but we can't find the person."

U.S. counterterrorism officials emphasized Friday that they have no evidence so far that the attack by Abdulazeez fell into that troubling security gap.

An unidentified Navy petty officer and a Chattanooga police officer were wounded. Abdulazeez was killed after exchanging gunfire with police.

While the FBI was cautious in making judgments, other lawmakers said there was clear reason to suspect that Abdulazeez had been inspired, directly or indirectly, by the Islamic State or a similar group.

"Based on my experience, I think he was radicalized by these individuals in Syria," Rep. Michael McCaul, R-Texas, chairman of the House Homeland Security Committee, told reporters. "The threat is real and it comes from the Internet," he added. "They don't have to travel to Iraq and Syria. . . . They're already here."

Abdulazeez could trace his heritage to several parts of the Middle East — he was born in Kuwait as a Jordanian citizen, though his parents identified themselves as Palestinians. He came to the United States with his family while still very young and grew up in Chattanooga, attending a local high school and earning a degree in electrical engineering from the University of Tennessee at Chattanooga.

In addition to his visit to Jordan last year, he traveled there on at least one prior occasion, during a combined trip to Kuwait in 2010, according to the official Kuwait News Agency.

A high school friend, Levon Miller, added that Abdulazeez traveled abroad once every few years. "He'd take off for a month or two mostly during his college breaks," Miller said, though he said he didn't know details about where he went.

Other signs emerged Friday that Abdulazeez and his four sisters had grown up in a troubled household, afflicted by marital strife and debt.

His father filed for federal bankruptcy protection in 2002. Seven years later, his mother filed for divorce, charging that her husband had sexually and physically abused her, and had threatened to take a second wife. The couple later reconciled.

Three months ago, he was hired as a shift supervisor by Superior Essex Inc., a firm that manufactures specialty wiring and cables. Co-workers said he called in sick last weekend and hadn't been seen since.

Video: After the shooting at a Chattanooga, Tenn., U.S. military recruitment center, Army Chief of Staff Gen. Ray Odierno said security at military recruiting and reserve centers will be reviewed. The Washington Post's Dan Lamothe describes previous attacks at military recruiting centers and why they are vulnerable targets. (Jason Aldag / The Washington Post)